Define the least count of an instrument.

How can you decrease the least count of a vernier callipers?

When is a vernier callipers said to be free from zero error?

Name the part of the vernier callipers which is used to measure the following —

1. External diameter of a tube,

2. Internal diameter of a mug,

3. Depth of a small bottle,

4. Thickness of a pencil.